1. Field of the Invention
The present invention relates to a metallic nanoparticle dispersion comprising a specific solvent as dispersion medium, and to metallic inks and pastes prepared thereof. The specific solvent confers to the dispersion improved stability in the absence of polymeric dispersants. The invention also relates to conductive layers and patterns formed with these metallic inks or pastes at moderate curing conditions.
2. Description of the Related Art
The interest in metallic printing or coating fluids, also referred to in this application as respectively a metallic ink or metallic coating solution, has increased during the last decades due to their unique properties when compared to the bulk properties of a given metal. For example, the melting point of metallic nanoparticles decreases with decreasing particle size making them of interest for printed electronics, electrochemical, optical, magnetic and biological applications.
The production of stable and concentrated metallic printing or coating fluids which can be printed, for example by inkjet printing, or coated at high speed is of great interest as it enables the preparation of electronic devices at low costs.
Usually, the main component of metallic printing or coating fluids is a metallic nanoparticle dispersion comprising metallic nanoparticles, a dispersant, typically a polymeric dispersant, and a dispersion medium. Such metallic nanoparticle dispersions can be directly used as a printing or coating fluid. However, additional ingredients are often added to the metallic nanoparticle dispersion to optimize the properties of the resulting metallic printing or coating fluids.
Usually, the preparation of metallic printing or coating fluids comprising metallic nanoparticles is carried out in water or organic solvents by the polyol synthesis methodology as disclosed in Mat. Chem. Phys. 114, 549-555, by a derivative of the polyol synthesis methodology or by an in-situ reduction of metallic salts in the presence of various reducing agents. Such methods are disclosed in for example US2010143591, US2009142482, US20060264518 and US20080220155, EP2147733, EP2139007, EP803551, EP2012952, EP2030706, EP1683592, EP166617, EP2119747, EP2087490 and EP2010314, WO2008/151066, WO2006/076603, WO2009/152388 and WO2009/157393.
Among others, the dilution of metallic nanoparticle dispersions, usually less than 1 wt % of metallic particles, is a severe drawback. Indeed, such highly diluted metallic nanoparticle dispersions cannot directly be used to prepare printing or coating fluids that require at least 5 wt % of metallic nanoparticles. An additional concentration step of the diluted metallic nanoparticle dispersion is then necessary before it can be used in the preparation of the printing or coating fluids.
WO2006/072959 discloses the production of silver nanoparticles compositions up to 35 wt % in water but the method still requires additional purification and isolation steps that impart drastically their industrialization and the scope of their applications.
A metallic nanoparticle dispersion typically comprises metallic nanoparticles, a dispersant, typically a polymeric dispersant, and a dispersion medium.
The presence of a polymeric dispersant is usually mandatory to obtain stable metallic printing or coating fluids. Non-stable metallic nanoparticle dispersions may lead to irreversible phase separation causing among others the clogging of the coating or print heads, which are usually only a few micrometers in diameter.
Polymeric dispersants typically contain in one part of the molecule so-called anchor groups, which adsorb onto the metallic particles to be dispersed. In another part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium and all the ingredients present in the final printing or coating fluids.
Polymeric dispersants are typically homo- or copolymers prepared from acrylic acid, methacrylic acid, vinyl pyrrolidinone, vinyl butyral, vinyl acetate or vinyl alcohol monomers.
Typically, after applying the metallic printing or coating fluids on a substrate, a sintering step, also referred to as curing step, at elevated temperatures is carried out to induce/enhance the conductivity of the applied patterns of layers. The organic components of the metallic printing or coating fluids, for example the polymeric dispersants, may reduce the sintering efficiency and thus the conductivity of the applied patterns of layers. For this reason, higher sintering temperatures and longer sintering times are often required to decompose the organic components.
Typical polymeric dispersants, such as those described above, are characterized by a full decomposition temperature of at least 350° C. Therefore, the layers or patterns coated or printed with metallic printing or coating fluids comprising such polymeric dispersants typically require a sintering step at elevated temperatures to be sure that most of the polymeric dispersants are decomposed.
Such high sintering temperatures are not compatible with common polymer foils, such as polyethylene terephthalate (PET) or polycarbonate, which have relatively low glass transition temperatures. This restricts the choice to more expensive polymers such as polyimide.
There is thus an interest in lowering the sintering temperatures needed to obtain conductive layers or patterns.
EP-A 10196244.7 filed on 21 Dec. 2010 discloses polymeric dispersants that has a 95 wt % decomposition at a temperature below 300° C. as measured by Thermal Gravimetric Analysis. By using metallic printing or coating fluids comprising such polymeric dispersants, the sintering temperature and time could be reduced.
In EP-A 11194791.7 and EP-A 11194790.9 both filed on 21 Dec. 2011 a so called sintering additive is used in combination with a polymeric dispersant of EP-A 10196244.7 to further lower the sintering temperature.